From NASA Astrophysics and Goddard Space Flight Center. Every day or two, on average, satellites detect a massive explosion somewhere in the sky. These are gamma-ray bursts, the brightest blasts in the universe. They’re thought to be caused by jets of matter moving near the speed of light associated with the births of black holes. Gamma-ray bursts that last longer than two seconds are the most common and are thought to result from the death of a massive star. Shorter bursts proved much more elusive.
In fact, even some of their basic properties were unknown until NASA’s Swift satellite began work in 2004. A neutron star is what remains when a star several times the mass of the sun collapses and explodes. With more than the sun’s mass packed in a sphere less than 18 miles across, these objects are incredibly dense. Just a sugar-cube-size piece of neutron star can weigh as much as all the water in the Great Lakes.
When two orbiting neutron stars collide, they merge and form a black hole, releasing enormous amounts of energy in the process. Armed with state-of-the-art supercomputer models, scientists have shown that colliding neutron stars can produce the energetic jet required for a gamma-ray burst. Earlier simulations demonstrated that mergers could make black holes. Others had shown that the high-speed particle jets needed to make a gamma-ray burst would continue if placed in the swirling wreckage of a recent merger.
Now, the simulations reveal the middle step of the process –how the merging stars’ magnetic field organizes itself into outwardly directed components capable of forming a jet. The Damiana supercomputer at Germany’s Max Planck Institute for Gravitational Physics needed six weeks to reveal the details of a process that unfolds in just 35 thousandths of a second. The new simulation shows two neutron stars merging to form a black hole surrounded by super-hot plasma.
On the left is a map of the density of the stars as they scramble their matter into a dense, hot cloud of swirling debris. On the right is a map of the magnetic fields, with blue representing magnetic strength a billion times greater than the sun’s. The simulation shows the same disorderly behavior of the matter and magnetic fields. Both structures gradually become more organized, but what’s important here is the white magnetic field. Amidst this incredible turmoil, the white field has taken on the character of a jet, although no matter is flowing through it when the simulation ends.
Showing that magnetic fields suddenly become organized as jets provides scientists with the missing link. It confirms that merging neutron stars can indeed produce short gamma-ray bursts. At this moment, somewhere across the cosmos, it’s about to happen again.